Lattice location and clustering of helium in ceramic oxides

Abstract

The lattice location and clustering of 3 He, implanted at 200 keV, in sapphire and magnesia have been investigated with ion channeling, nuclear reaction analysis and transmission electron microscopy. A typical helium atom implanted in sapphire at 300 K prefers to locate near an octahedral interstice that was intrinsically unfilled by a cation, known as a constitutional vacancy. The occupation of this lattice site dominates over either substitutional placement in irradiation-produced vacancies or aggregation in bubbles. The fraction of implanted helium occupying a unique lattice site decreases for irradiation temperatures greater than about 800 K and becomes negligible above 1073 K. In sapphire, cavities decorating dislocation loops were observed to form for implantation at 1073 K. The inability to assign a definite lattice site for helium in magnesia can be attributed the absence of constitutional vacancies within the crystal structure.